Conventional articles of footwear, including athletic footwear, have included two primary elements, namely an upper member and a sole structure. The upper member provides a covering for the foot that securely receives and positions the foot with respect to the sole structure. In addition, the upper member may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole structure generally is secured to a lower portion of the upper member and generally is positioned between the foot and a contact surface (which may include any foot or footwear contact surface, including but not limited to: ground, grass, dirt, sand, snow, ice, tile, flooring, carpeting, synthetic grass, artificial turf, and the like). In addition to attenuating contact surface reaction forces, the sole structure may provide traction and help control foot motion, such as pronation. Accordingly, the upper member and the sole structure operate cooperatively to provide a comfortable structure that is suited for a variety of ambulatory activities, such as walking and running.
The sole structure of athletic footwear, in at least some instances, will exhibit a layered configuration that includes a comfort-enhancing insole, a resilient midsole (e.g., formed, at least in part, from a polymer foam material), and a contact surface-contacting outsole that provides both abrasion-resistance and traction. The midsole, in at least some instances, will be the primary sole structure element that attenuates contact surface reaction forces and controls foot motion. Suitable polymer foam materials for at least portions of the midsole include ethylvinylacetate (“EVA”) or polyurethane (“PU”) that compress resiliently under an applied load to attenuate contact surface reaction forces. Conventional polymer foam materials are resiliently compressible, in part, due to the inclusion of a plurality of open or closed cells that define an inner volume substantially displaced by gas.